The present invention relates to light strings such as are used for holiday lighting, and in particular to mechanical shunts for passing electrical current to the next light in the string if a bulb is loose or missing.
Strings of lights are typically wired electrically in series. Consequently, when one light in the string burns out or is removed, all the lights in the string go out. Determining which light needs to be replaced is somewhat tedious. If the string has 50 or more lights and the string is attached to a Christmas tree, finding the burned out or missing bulb can be very tedious.
For a number of years, this problem has been solved, or at least avoided, by the use of shunts that allow current to pass directly between the terminals of the defective lamp, bypassing the missing or defective bulb filament. Passing electrical current from one lamp to the next regardless of the condition of the bulb in any individual lamp allows the remaining lamps to continue to operate.
Shunts are typically found in two places in prior art lamps, namely, in the glass bulb and in the socket. The shunts inside the glass bulb are typically coils of wire wrapped around the conductive elements (called Dumet wires). When the filament fails, the oxide coating on the wires that theretofore prevented direct conduction of electricity is burned off and the coil welds itself to the Dumet wires, thereby providing a new electrically conductive path for passing the electrical current.
Of the shunts that are located in the socket, there are two types, namely, solid state shunts and mechanical shunts. Among the mechanical shunts, for example, there is a set of spring contact terminals that is the subject of U.S. Pat. No. 6,257,740. These spring contacts are pushed apart when the lamp base is inserted into the socket and spring back together when the base is removed, thereby allowing the current to pass from one terminal to the other directly. This type is strictly for use when the bulb (and its base) is removed and does not address the issue of a burned out bulb. This type of shunt works well and has enjoyed commercial success.
Another mechanical shunt is disclosed in U.S. Pat. No. 7,253,556, which is invented by one of the present inventors and is commonly owned by applicant. This mechanical shunt is a nearly horizontal flat strip of metal held in place between the two electrical terminals in a light socket by a shunt holder. The ends of the shunt extend laterally and slightly downwardly to engage the electrical terminals mounted to the socket wall. When the lamp base, which is hollow, is inserted into the socket, the shunt holder together with its shunt is received inside the hollow base, and, as the shunt enters the base, its lateral ends are bent down and away from the electrical terminals on the socket wall, thereby allowing electrical current to pass to and through the Dumet wires and thence to the filament in the bulb rather than directly through the shunt between the electrical terminals.
U.S. Pat. No. 6,609,814 issued to Ahroni teaches a mechanical shunt composed of two portions. The two portions of the shunt are nearly co-planar and in electric contact with each other and the electrical terminals mounted on the wall of his socket. When the bulb holder is inserted into a socket, a first of the two portions is pressed down, out of engagement and electrical connection with the second of the two portions. When the bulb holder is removed from the socket, the first portion resiliently resumes electrical contact with the second portion.
One problem with miniature lamps is the corrosion of electrical conductors. There are two main contributors to corrosion in lamps. The first is the intrusion of moisture from the environment. The second is the release of gases from the plastic, which gases accelerate the process of natural corrosion. The build up of corrosion quickly renders electrical conductors non-conductive. In a mechanical shunt, a corroded shunt will cause a light string failure as surely as the lack of a shunt.
There are a huge number of light strings manufactured and sold each year throughout the world. The number is so large that even small changes that, for example, reduce material requirements, simplify manufacturing, or improve safety or reliability, make a huge difference in the costs to manufacture. Accordingly, there remains a need for a better mechanical shunt for use in the sockets of the lamps of light strings.